1. Field of the Invention
This invention relates to fail-fixed servosystems, and particularly to a new and improved servosystem in which a variation from a predetermined range of electrical input results in fixing-in-position of the system output, through either hydraulic or mechanical locking means, with means effective after fixing-in-position for providing controlled drift and manual adjustment of the output.
2. Description of the Prior Art
Electrohydraulic servovalves are widely used as interface devices between electrical control systems and mechanical or hydraulic metering or actuating devices. For example, in a gas turbine engine fuel control system, an electrical control signal generated by an electronic fuel control computer may be applied to the input of a servovalve. In response to the electrical input signal, the servovalve controls the movement of a servoposition which translates within a bore to generate a mechanical output signal which through a series of mechanical linkages varies the position of a mechanical fuel metering valve. Thus, the flow of fuel to the gas turbine engine, and therefore other engine parameters such as rotational speeds, can be accurately controlled as a function of the computer generated electrical signal.
Due to the widespread use of such servovalves in critical control systems, such as the above-described gas turbine engine fuel control system, it is desirable for the servosystem to be fail-fixed. By a fail-fixed servosystem it is meant that the mechanical output of the servovalve is locked, or fixed in position, in the event the electrical input signal varies from a predetermined range of values. The variation can be either a total loss of electrical input or an input which is above or below the predetermined range. The terms "fixing-in-position" or "fixed in position", therefore, mean that an element of a system, such as its output, is locked in position.
Most currently available fail-fixed electrohydraulic servosystems rely on electrical or hydromechanical feedback to provide an error signal to the electrohydraulic servo before the fail-fixed feature will operate, and thus introduce a potential unreliability factor in the feedback system with attendant uncontrolled and thus undesirable drifts of system output. Current systems do not provide for a controlled drift capability of the servosystem output after it is fixed in position. Controlled drift is an important feature in that it permits the output of the system to be automatically adjusted at a predetermined rate and in a predetermined direction from its fail-fixed position to a position which is considered desirable for safety and efficiency reasons. Additionally, most available arrangements do not permit manual adjustment of the system output after it is fixed in position. Where the servosystem is incorporated on an aircraft engine, for example, emergency temporary manual adjustment of a fixed in position component may be important in order to return the aircraft to a location where major repairs can be completed.
A primary object of the present invention is, therefore, to provide a new and improved fail-fixed electrohydraulic servosystem which will simply and reliably positively lock, or fix in position, the output of a servosystem in the event that the electrical input to the servosystem is lost or varies from a predetermined range of values.
Another object of the present invention is to provide a fail-fixed electrohydraulic servosystem with a controlled drift capability effective following fixing-in-position.
Still another object of the present invention is to provide a fail-fixed electrohydraulic servosystem with a means for enabling manual adjustment of the output after fixing-in-position.